Journal of a Walk Down the River Thames

by Keith Pauling

Alien Invaders

The parakeets are attractive and as far as I am aware do not damage the environment that has contributed to their present numbers. Oh that the same could be said of other species that have made their home here and prospered. I am in the presence of unwanted invaders.

They have been here since 1935, but in the last twenty years they have “bred and spread” up the tidal Thames and are believed to be as far up the river as Staines. The Environment Agency believes that there are tens of thousands of the unwanted invaders causing severe environmental damage to the riverbanks.

The name of this invader of our shores is Eriocheir sinensis more commonly known as the Chinese Mitten Crab. It is a medium-sized crab native to South-East Asia. It takes its common name from the dense hairs on its claws that give the impression it is wearing mittens. The body is about the size of the human hand, typically 30 to 100mm across, with its legs approximately twice the length that the body is wide.

The crab probably came to our shores in the holds of ships, and they are now present in the Humber system and the Tyne, and are also becoming an increasing problem on the Rhine and in North West America. If you see a crab in freshwater anywhere in Britain it is almost certainly a mitten crab.

The mitten crab spends most of its life-cycle in freshwater, returning to the sea to breed. When they reach four or five years of age they begin their migration to the sea in late summer and then do whatever it is that mitten crabs do in the estuary waters. After mating the female continues to deeper waters for the winter and returns to the protection of the estuary to lay her eggs. The eggs hatch as larvae, and change into the young crabs which then move upstream in search of freshwater in which to grow.

The environmental damage caused by the severe erosion of the banks as the crabs burrow into them. Not only that, but the damage done to the freshwater fish stocks can be considerable because the crabs eat the fish eggs laid on the river bottom.

In China the crab is considered a great delicacy, but over here they are simply a menace. In common with many shellfish the mitten crab has a great tolerance to polluted environments and wild crabs can be very hazardous to eat.